The electron microscope (EM) has been a mainstay for study of fastidious gastroenteritis viruses. Despite the development of 2nd and 3rd generation tests, EM remains an essential tool: (1) as the "supreme court" when new tests yield variable results; (2) in the search for new agents of viral gastroenteritis; (3) for visualizing the site of attachment of antibody to the virion in antigen-antibody reactions; (4) for serologic studies; (5)\for direct visualization and fine structure characterization of the morphology of virus particles; and (6) for studying specimens derived from individuals with diseases of unknown etiology such as non-A, non-B hepatitis by immune electron microscopy. A new technique was introduced during the past year - solid phase immune electron microscopy - which was used successfully for the rapid serotyping of human and animal rotaviruses. In addition, the technique appeared to be more sensitive than conventional immune electron microscopy for rotavirus detection. Finally, since over 50% of the episodes of pediatric diarrhea are without known etiology, EM should continue to prove to be a valuable tool in the search for such agents.